Clairvoyance really means "to have clear vision" and is generally defined as "gaining information through a means other than the known human senses." This gives it a lot of scope, from people who see visions, dream prophecies or people who simply get a gut-feeling.
From a scientific perspective, our brains are receiving so much information each second and our brains are constantly adjusting our perspective to something we can cope with (look up practically any optical illusion). Research into the field of parapsychology suggests that many people who claim to be psychic or clairvoyant show a particular pattern of brain activity which is quite similar to the brain activity of people who suffer from epilepsy. It is therefore easy for many people to reduce clairvoyancy to hallucinations.
I think it's a little more complicated than that. The only reason why it is a pseudoscience and not a science is because the field has not been given the resources (financially and intellectually) to be a science. There is however data out there and more data to be recorded and analysed.
I would say that I have clairvoyant moments but a lot of people do (whether they want to call it clairvoyance, intuition or whatever) so I wouldn't necessarily call myself a clairvoyant. At the end of the day, what people call themselves and how they define their abilities are so varied and so personal that there is no set standard or even a scale to measure against.